Brushless or electronically commutated motors have long been used in industrial applications and are making an entrance into automotive applications. Common topologies used for such motors are 3 phase motors, both unipolar and bipolar. These topologies are well suited to lower power applications.
In the case of the three phase unipolar motor, one phase is switched on at any given time. As a result, 33% of the copper is utilized at any given time. Furthermore, one must deal with the use issue of the energy recovery during the “off” portion of the pulse width modulation. Inverter topologies such as the R-dump and C-dump effectively deal with the recovery of energy to the positive rail during the off portions of the pulse width modulation, while at the same time, prevent the conduction of the motor back EMF through the energy recovery apparatus.
In the case of a wye connected, three phase bipolar motor with 120 degree commutation, two phases are on at any given time. In a wye connected three phase motor the current flows through two series transistors and through the two series windings resulting in 66 percent copper utilization at any given time. Special energy recovery devices in this topology are not required as energy recovery is achieved through the inherent drain to source diodes in the inverter legs.
In both the three phase unipolar and bipolar case, without a special costly heatsinking arrangement, the maximum attainable motor power will be limited.
Accordingly, there is a need to increase the maximum power levels attainable and to make the most efficient use of the current in a brushless motor.